Updating to New Releases

create-react-app is a global command-line utility that you use to create new projects.

react-scripts is a development dependency in the generated projects (including this one).

You almost never need to update create-react-app itself: it delegates all the setup to react-scripts.

When you run create-react-app, it always creates the project with the latest version of react-scripts so you’ll get all the new features and improvements in newly created apps automatically.

To update an existing project to a new version of react-scripts, open the changelog, find the version you’re currently on (check package.json in this folder if you’re not sure), and apply the migration instructions for the newer versions.

In most cases bumping the react-scripts version in package.json and running npm install in this folder should be enough, but it’s good to consult the changelog for potential breaking changes.

We commit to keeping the breaking changes minimal so you can upgrade react-scripts painlessly.

npm run eject

If you aren’t satisfied with the build tool and configuration choices, you can eject at any time. This command will remove the single build dependency from your project.

Instead, it will copy all the configuration files and the transitive dependencies (Webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except eject will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.

You don’t have to ever use eject. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.

While we recommend using experimental proposals with some caution, Facebook heavily uses these features in the product code, so we intend to provide codemods if any of these proposals change in the future.

If you use any other ES6+ features that need runtime support (such as Array.from() or Symbol), make sure you are including the appropriate polyfills manually, or that the browsers you are targeting already support them.

Also note that using some newer syntax features like for...of or [...nonArrayValue] causes Babel to emit code that depends on ES6 runtime features and might not work without a polyfill. When in doubt, use Babel REPL to see what any specific syntax compiles down to.

Syntax Highlighting in the Editor

To configure the syntax highlighting in your favorite text editor, head to the relevant Babel documentation page and follow the instructions. Some of the most popular editors are covered.

Displaying Lint Output in the Editor

Note: this feature is available with react-scripts@0.2.0 and higher.
It also only works with npm 3 or higher.

They are not required for linting. You should see the linter output right in your terminal as well as the browser console. However, if you prefer the lint results to appear right in your editor, there are some extra steps you can do.

You would need to install an ESLint plugin for your editor first. Then, add a file called .eslintrc to the project root:

{
"extends":"react-app"
}

Now your editor should report the linting warnings.

Note that even if you edit your .eslintrc file further, these changes will only affect the editor integration. They won’t affect the terminal and in-browser lint output. This is because Create React App intentionally provides a minimal set of rules that find common mistakes.

If you want to enforce a coding style for your project, consider using Prettier instead of ESLint style rules.

Debugging in the Editor

Visual Studio Code and WebStorm support debugging out of the box with Create React App. This enables you as a developer to write and debug your React code without leaving the editor, and most importantly it enables you to have a continuous development workflow, where context switching is minimal, as you don’t have to switch between tools.

Start your app by running npm start, and start debugging in VS Code by pressing F5 or by clicking the green debug icon. You can now write code, set breakpoints, make changes to the code, and debug your newly modified code—all from your editor.

Start your app by running npm start, then press ^D on macOS or F9 on Windows and Linux or click the green debug icon to start debugging in WebStorm.

The same way you can debug your application in IntelliJ IDEA Ultimate, PhpStorm, PyCharm Pro, and RubyMine.

Formatting Code Automatically

Prettier is an opinionated code formatter with support for JavaScript, CSS and JSON. With Prettier you can format the code you write automatically to ensure a code style within your project. See the Prettier's GitHub page for more information, and look at this page to see it in action.

To format our code whenever we make a commit in git, we need to install the following dependencies:

Now, whenever you make a commit, Prettier will format the changed files automatically. You can also run ./node_modules/.bin/prettier --single-quote --write "src/**/*.{js,jsx,json,css}" to format your entire project for the first time.

Next you might want to integrate Prettier in your favorite editor. Read the section on Editor Integration on the Prettier GitHub page.

Changing the Page <title>

You can find the source HTML file in the public folder of the generated project. You may edit the <title> tag in it to change the title from “React App” to anything else.

Note that normally you wouldn’t edit files in the public folder very often. For example, adding a stylesheet is done without touching the HTML.

If you need to dynamically update the page title based on the content, you can use the browser document.title API. For more complex scenarios when you want to change the title from React components, you can use React Helmet, a third party library.

If you use a custom server for your app in production and want to modify the title before it gets sent to the browser, you can follow advice in this section. Alternatively, you can pre-build each page as a static HTML file which then loads the JavaScript bundle, which is covered here.

Installing a Dependency

The generated project includes React and ReactDOM as dependencies. It also includes a set of scripts used by Create React App as a development dependency. You may install other dependencies (for example, React Router) with npm:

npm install --save react-router

Alternatively you may use yarn:

yarn add react-router

This works for any library, not just react-router.

Importing a Component

This project setup supports ES6 modules thanks to Babel.
While you can still use require() and module.exports, we encourage you to use import and export instead.

We suggest that you stick to using default imports and exports when a module only exports a single thing (for example, a component). That’s what you get when you use export default Button and import Button from './Button'.

Named exports are useful for utility modules that export several functions. A module may have at most one default export and as many named exports as you like.

Code Splitting

Instead of downloading the entire app before users can use it, code splitting allows you to split your code into small chunks which you can then load on demand.

This project setup supports code splitting via dynamic import(). Its proposal is in stage 3. The import() function-like form takes the module name as an argument and returns a Promise which always resolves to the namespace object of the module.

Adding a Stylesheet

This project setup uses Webpack for handling all assets. Webpack offers a custom way of “extending” the concept of import beyond JavaScript. To express that a JavaScript file depends on a CSS file, you need to import the CSS from the JavaScript file:

This is not required for React but many people find this feature convenient. You can read about the benefits of this approach here. However you should be aware that this makes your code less portable to other build tools and environments than Webpack.

In development, expressing dependencies this way allows your styles to be reloaded on the fly as you edit them. In production, all CSS files will be concatenated into a single minified .css file in the build output.

If you are concerned about using Webpack-specific semantics, you can put all your CSS right into src/index.css. It would still be imported from src/index.js, but you could always remove that import if you later migrate to a different build tool.

Post-Processing CSS

This project setup minifies your CSS and adds vendor prefixes to it automatically through Autoprefixer so you don’t need to worry about it.

Adding a CSS Preprocessor (Sass, Less etc.)

Generally, we recommend that you don’t reuse the same CSS classes across different components. For example, instead of using a .Button CSS class in <AcceptButton> and <RejectButton> components, we recommend creating a <Button> component with its own .Button styles, that both <AcceptButton> and <RejectButton> can render (but not inherit).

Following this rule often makes CSS preprocessors less useful, as features like mixins and nesting are replaced by component composition. You can, however, integrate a CSS preprocessor if you find it valuable. In this walkthrough, we will be using Sass, but you can also use Less, or another alternative.

Note: To use a different preprocessor, replace build-css and watch-css commands according to your preprocessor’s documentation.

Now you can rename src/App.css to src/App.scss and run npm run watch-css. The watcher will find every Sass file in src subdirectories, and create a corresponding CSS file next to it, in our case overwriting src/App.css. Since src/App.js still imports src/App.css, the styles become a part of your application. You can now edit src/App.scss, and src/App.css will be regenerated.

To share variables between Sass files, you can use Sass imports. For example, src/App.scss and other component style files could include @import "./shared.scss"; with variable definitions.

To enable importing files without using relative paths, you can add the --include-path option to the command in package.json.

At this point you might want to remove all CSS files from the source control, and add src/**/*.css to your .gitignore file. It is generally a good practice to keep the build products outside of the source control.

As a final step, you may find it convenient to run watch-css automatically with npm start, and run build-css as a part of npm run build. You can use the && operator to execute two scripts sequentially. However, there is no cross-platform way to run two scripts in parallel, so we will install a package for this:

npm install --save npm-run-all

Alternatively you may use yarn:

yarn add npm-run-all

Then we can change start and build scripts to include the CSS preprocessor commands:

node-sass has been reported as having issues with detecting new files in a directory #1891

node-sass-chokidar is used here as it addresses these issues.

Adding Images, Fonts, and Files

With Webpack, using static assets like images and fonts works similarly to CSS.

You can import a file right in a JavaScript module. This tells Webpack to include that file in the bundle. Unlike CSS imports, importing a file gives you a string value. This value is the final path you can reference in your code, e.g. as the src attribute of an image or the href of a link to a PDF.

To reduce the number of requests to the server, importing images that are less than 10,000 bytes returns a data URI instead of a path. This applies to the following file extensions: bmp, gif, jpg, jpeg, and png. SVG files are excluded due to #1153.

This ensures that when the project is built, Webpack will correctly move the images into the build folder, and provide us with correct paths.

This works in CSS too:

.Logo {
background-image: url(./logo.png);
}

Webpack finds all relative module references in CSS (they start with ./) and replaces them with the final paths from the compiled bundle. If you make a typo or accidentally delete an important file, you will see a compilation error, just like when you import a non-existent JavaScript module. The final filenames in the compiled bundle are generated by Webpack from content hashes. If the file content changes in the future, Webpack will give it a different name in production so you don’t need to worry about long-term caching of assets.

Please be advised that this is also a custom feature of Webpack.

It is not required for React but many people enjoy it (and React Native uses a similar mechanism for images).
An alternative way of handling static assets is described in the next section.

Using the public Folder

Note: this feature is available with react-scripts@0.5.0 and higher.

Changing the HTML

The public folder contains the HTML file so you can tweak it, for example, to set the page title.
The <script> tag with the compiled code will be added to it automatically during the build process.

Result filenames include content hashes so you don’t need to worry about browsers caching their old versions.

However there is an escape hatch that you can use to add an asset outside of the module system.

If you put a file into the public folder, it will not be processed by Webpack. Instead it will be copied into the build folder untouched. To reference assets in the public folder, you need to use a special variable called PUBLIC_URL.

Inside index.html, you can use it like this:

<linkrel="shortcut icon"href="%PUBLIC_URL%/favicon.ico">

Only files inside the public folder will be accessible by %PUBLIC_URL% prefix. If you need to use a file from src or node_modules, you’ll have to copy it there to explicitly specify your intention to make this file a part of the build.

When you run npm run build, Create React App will substitute %PUBLIC_URL% with a correct absolute path so your project works even if you use client-side routing or host it at a non-root URL.

In JavaScript code, you can use process.env.PUBLIC_URL for similar purposes:

render() {
// Note: this is an escape hatch and should be used sparingly!// Normally we recommend using `import` for getting asset URLs// as described in “Adding Images and Fonts” above this section.return<img src={process.env.PUBLIC_URL+'/img/logo.png'} />;
}

Keep in mind the downsides of this approach:

None of the files in public folder get post-processed or minified.

Missing files will not be called at compilation time, and will cause 404 errors for your users.

Result filenames won’t include content hashes so you’ll need to add query arguments or rename them every time they change.

When to Use the public Folder

Normally we recommend importing stylesheets, images, and fonts from JavaScript.
The public folder is useful as a workaround for a number of less common cases:

You have thousands of images and need to dynamically reference their paths.

You want to include a small script like pace.js outside of the bundled code.

Some library may be incompatible with Webpack and you have no other option but to include it as a <script> tag.

Note that if you add a <script> that declares global variables, you also need to read the next section on using them.

Using Global Variables

When you include a script in the HTML file that defines global variables and try to use one of these variables in the code, the linter will complain because it cannot see the definition of the variable.

You can avoid this by reading the global variable explicitly from the window object, for example:

const$=window.$;

This makes it obvious you are using a global variable intentionally rather than because of a typo.

Alternatively, you can force the linter to ignore any line by adding // eslint-disable-line after it.

Adding Bootstrap

You don’t have to use React Bootstrap together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:

Install React Bootstrap and Bootstrap from npm. React Bootstrap does not include Bootstrap CSS so this needs to be installed as well:

Adding Flow

Recent versions of Flow work with Create React App projects out of the box.

To add Flow to a Create React App project, follow these steps:

Run npm install --save flow-bin (or yarn add flow-bin).

Add "flow": "flow" to the scripts section of your package.json.

Run npm run flow init (or yarn flow init) to create a .flowconfig file in the root directory.

Add // @flow to any files you want to type check (for example, to src/App.js).

Now you can run npm run flow (or yarn flow) to check the files for type errors.
You can optionally use an IDE like Nuclide for a better integrated experience.
In the future we plan to integrate it into Create React App even more closely.

Adding Custom Environment Variables

Note: this feature is available with react-scripts@0.2.3 and higher.

Your project can consume variables declared in your environment as if they were declared locally in your JS files. By
default you will have NODE_ENV defined for you, and any other environment variables starting with
REACT_APP_.

The environment variables are embedded during the build time. Since Create React App produces a static HTML/CSS/JS bundle, it can’t possibly read them at runtime. To read them at runtime, you would need to load HTML into memory on the server and replace placeholders in runtime, just like described here. Alternatively you can rebuild the app on the server anytime you change them.

These environment variables will be defined for you on process.env. For example, having an environment
variable named REACT_APP_SECRET_CODE will be exposed in your JS as process.env.REACT_APP_SECRET_CODE.

There is also a special built-in environment variable called NODE_ENV. You can read it from process.env.NODE_ENV. When you run npm start, it is always equal to 'development', when you run npm test it is always equal to 'test', and when you run npm run build to make a production bundle, it is always equal to 'production'. You cannot override NODE_ENV manually. This prevents developers from accidentally deploying a slow development build to production.

These environment variables can be useful for displaying information conditionally based on where the project is
deployed or consuming sensitive data that lives outside of version control.

First, you need to have environment variables defined. For example, let’s say you wanted to consume a secret defined
in the environment inside a <form>:

During the build, process.env.REACT_APP_SECRET_CODE will be replaced with the current value of the REACT_APP_SECRET_CODE environment variable. Remember that the NODE_ENV variable will be set for you automatically.

When you load the app in the browser and inspect the <input>, you will see its value set to abcdef, and the bold text will show the environment provided when using npm start:

The above form is looking for a variable called REACT_APP_SECRET_CODE from the environment. In order to consume this
value, we need to have it defined in the environment. This can be done using two ways: either in your shell or in
a .env file. Both of these ways are described in the next few sections.

Having access to the NODE_ENV is also useful for performing actions conditionally:

if (process.env.NODE_ENV!=='production') {
analytics.disable();
}

When you compile the app with npm run build, the minification step will strip out this condition, and the resulting bundle will be smaller.

Referencing Environment Variables in the HTML

Note: this feature is available with react-scripts@0.9.0 and higher.

You can also access the environment variables starting with REACT_APP_ in the public/index.html. For example:

<title>%REACT_APP_WEBSITE_NAME%</title>

Note that the caveats from the above section apply:

Apart from a few built-in variables (NODE_ENV and PUBLIC_URL), variable names must start with REACT_APP_ to work.

These variables will act as the defaults if the machine does not explicitly set them.
Please refer to the dotenv documentation for more details.

Note: If you are defining environment variables for development, your CI and/or hosting platform will most likely need
these defined as well. Consult their documentation how to do this. For example, see the documentation for Travis CI or Heroku.

Create React App will add decorator support when the specification advances to a stable stage.

Fetching Data with AJAX Requests

React doesn't prescribe a specific approach to data fetching, but people commonly use either a library like axios or the fetch() API provided by the browser. Conveniently, Create React App includes a polyfill for fetch() so you can use it without worrying about the browser support.

The global fetch function allows to easily makes AJAX requests. It takes in a URL as an input and returns a Promise that resolves to a Response object. You can find more information about fetchhere.

This project also includes a Promise polyfill which provides a full implementation of Promises/A+. A Promise represents the eventual result of an asynchronous operation, you can find more information about Promises here and here. Both axios and fetch() use Promises under the hood. You can also use the async / await syntax to reduce the callback nesting.

Such setup is not required. However, if you do have a setup like this, it is convenient to write requests like fetch('/api/todos') without worrying about redirecting them to another host or port during development.

To tell the development server to proxy any unknown requests to your API server in development, add a proxy field to your package.json, for example:

"proxy":"http://localhost:4000",

This way, when you fetch('/api/todos') in development, the development server will recognize that it’s not a static asset, and will proxy your request to http://localhost:4000/api/todos as a fallback. The development server will only attempt to send requests without text/html in its Accept header to the proxy.

Conveniently, this avoids CORS issues and error messages like this in development:

Fetch API cannot load http://localhost:4000/api/todos. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

Keep in mind that proxy only has effect in development (with npm start), and it is up to you to ensure that URLs like /api/todos point to the right thing in production. You don’t have to use the /api prefix. Any unrecognized request without a text/html accept header will be redirected to the specified proxy.

The proxy option supports HTTP, HTTPS and WebSocket connections.
If the proxy option is not flexible enough for you, alternatively you can:

"Invalid Host Header" Errors After Configuring Proxy

When you enable the proxy option, you opt into a more strict set of host checks. This is necessary because leaving the backend open to remote hosts makes your computer vulnerable to DNS rebinding attacks. The issue is explained in this article and this issue.

This shouldn’t affect you when developing on localhost, but if you develop remotely like described here, you will see this error in the browser after enabling the proxy option:

Invalid Host header

To work around it, you can specify your public development host in a file called .env.development in the root of your project:

HOST=mypublicdevhost.com

If you restart the development server now and load the app from the specified host, it should work.

If you are still having issues or if you’re using a more exotic environment like a cloud editor, you can bypass the host check completely by adding a line to .env.development.local. Note that this is dangerous and exposes your machine to remote code execution from malicious websites:

# NOTE: THIS IS DANGEROUS!
# It exposes your machine to attacks from the websites you visit.
DANGEROUSLY_DISABLE_HOST_CHECK=true

We don’t recommend this approach.

Configuring the Proxy Manually

Note: this feature is available with react-scripts@1.0.0 and higher.

If the proxy option is not flexible enough for you, you can specify an object in the following form (in package.json).
You may also specify any configuration value http-proxy-middleware or http-proxy supports.

Configuring a WebSocket Proxy

When setting up a WebSocket proxy, there are a some extra considerations to be aware of.

If you’re using a WebSocket engine like Socket.io, you must have a Socket.io server running that you can use as the proxy target. Socket.io will not work with a standard WebSocket server. Specifically, don't expect Socket.io to work with the websocket.org echo test.

Using HTTPS in Development

Note: this feature is available with react-scripts@0.4.0 and higher.

You may require the dev server to serve pages over HTTPS. One particular case where this could be useful is when using the "proxy" feature to proxy requests to an API server when that API server is itself serving HTTPS.

To do this, set the HTTPS environment variable to true, then start the dev server as usual with npm start:

Windows (cmd.exe)

setHTTPS=true&&npm start

Windows (Powershell)

($env:HTTPS=$true) -and (npm start)

(Note: the lack of whitespace is intentional.)

Linux, macOS (Bash)

HTTPS=true npm start

Note that the server will use a self-signed certificate, so your web browser will almost definitely display a warning upon accessing the page.

Generating Dynamic <meta> Tags on the Server

Since Create React App doesn’t support server rendering, you might be wondering how to make <meta> tags dynamic and reflect the current URL. To solve this, we recommend to add placeholders into the HTML, like this:

Then, on the server, regardless of the backend you use, you can read index.html into memory and replace __OG_TITLE__, __OG_DESCRIPTION__, and any other placeholders with values depending on the current URL. Just make sure to sanitize and escape the interpolated values so that they are safe to embed into HTML!

If you use a Node server, you can even share the route matching logic between the client and the server. However duplicating it also works fine in simple cases.

Pre-Rendering into Static HTML Files

If you’re hosting your build with a static hosting provider you can use react-snapshot or react-snap to generate HTML pages for each route, or relative link, in your application. These pages will then seamlessly become active, or “hydrated”, when the JavaScript bundle has loaded.

There are also opportunities to use this outside of static hosting, to take the pressure off the server when generating and caching routes.

The primary benefit of pre-rendering is that you get the core content of each page with the HTML payload—regardless of whether or not your JavaScript bundle successfully downloads. It also increases the likelihood that each route of your application will be picked up by search engines.

Then, on the server, you can replace __SERVER_DATA__ with a JSON of real data right before sending the response. The client code can then read window.SERVER_DATA to use it. Make sure to sanitize the JSON before sending it to the client as it makes your app vulnerable to XSS attacks.

Running Tests

Create React App uses Jest as its test runner. To prepare for this integration, we did a major revamp of Jest so if you heard bad things about it years ago, give it another try.

Jest is a Node-based runner. This means that the tests always run in a Node environment and not in a real browser. This lets us enable fast iteration speed and prevent flakiness.

While Jest provides browser globals such as window thanks to jsdom, they are only approximations of the real browser behavior. Jest is intended to be used for unit tests of your logic and your components rather than the DOM quirks.

We recommend that you use a separate tool for browser end-to-end tests if you need them. They are beyond the scope of Create React App.

Filename Conventions

Jest will look for test files with any of the following popular naming conventions:

Files with .js suffix in __tests__ folders.

Files with .test.js suffix.

Files with .spec.js suffix.

The .test.js / .spec.js files (or the __tests__ folders) can be located at any depth under the src top level folder.

We recommend to put the test files (or __tests__ folders) next to the code they are testing so that relative imports appear shorter. For example, if App.test.js and App.js are in the same folder, the test just needs to import App from './App' instead of a long relative path. Colocation also helps find tests more quickly in larger projects.

Command Line Interface

When you run npm test, Jest will launch in the watch mode. Every time you save a file, it will re-run the tests, just like npm start recompiles the code.

The watcher includes an interactive command-line interface with the ability to run all tests, or focus on a search pattern. It is designed this way so that you can keep it open and enjoy fast re-runs. You can learn the commands from the “Watch Usage” note that the watcher prints after every run:

Version Control Integration

By default, when you run npm test, Jest will only run the tests related to files changed since the last commit. This is an optimization designed to make your tests run fast regardless of how many tests you have. However it assumes that you don’t often commit the code that doesn’t pass the tests.

Jest will always explicitly mention that it only ran tests related to the files changed since the last commit. You can also press a in the watch mode to force Jest to run all tests.

Jest will always run all tests on a continuous integration server or if the project is not inside a Git or Mercurial repository.

Writing Tests

To create tests, add it() (or test()) blocks with the name of the test and its code. You may optionally wrap them in describe() blocks for logical grouping but this is neither required nor recommended.

Jest provides a built-in expect() global function for making assertions. A basic test could look like this:

Testing Components

There is a broad spectrum of component testing techniques. They range from a “smoke test” verifying that a component renders without throwing, to shallow rendering and testing some of the output, to full rendering and testing component lifecycle and state changes.

Different projects choose different testing tradeoffs based on how often components change, and how much logic they contain. If you haven’t decided on a testing strategy yet, we recommend that you start with creating simple smoke tests for your components:

This test mounts a component and makes sure that it didn’t throw during rendering. Tests like this provide a lot of value with very little effort so they are great as a starting point, and this is the test you will find in src/App.test.js.

When you encounter bugs caused by changing components, you will gain a deeper insight into which parts of them are worth testing in your application. This might be a good time to introduce more specific tests asserting specific expected output or behavior.

If you’d like to test components in isolation from the child components they render, we recommend using shallow() rendering API from Enzyme. To install it, run:

npm install --save enzyme enzyme-adapter-react-16 react-test-renderer

Alternatively you may use yarn:

yarn add enzyme enzyme-adapter-react-16 react-test-renderer

As of Enzyme 3, you will need to install Enzyme along with an Adapter corresponding to the version of React you are using. (The examples above use the adapter for React 16.)

src/setupTests.js

Note: Keep in mind that if you decide to "eject" before creating src/setupTests.js, the resulting package.json file won't contain any reference to it. Read here to learn how to add this after ejecting.

Unlike the previous smoke test using ReactDOM.render(), this test only renders <App> and doesn’t go deeper. For example, even if <App> itself renders a <Button> that throws, this test will pass. Shallow rendering is great for isolated unit tests, but you may still want to create some full rendering tests to ensure the components integrate correctly. Enzyme supports full rendering with mount(), and you can also use it for testing state changes and component lifecycle.

You can read the Enzyme documentation for more testing techniques. Enzyme documentation uses Chai and Sinon for assertions but you don’t have to use them because Jest provides built-in expect() and jest.fn() for spies.

Here is an example from Enzyme documentation that asserts specific output, rewritten to use Jest matchers:

However, if you are used to other libraries, such as Chai and Sinon, or if you have existing code using them that you’d like to port over, you can import them normally like this:

importsinonfrom'sinon';
import { expect } from'chai';

and then use them in your tests like you normally do.

Initializing Test Environment

Note: this feature is available with react-scripts@0.4.0 and higher.

If your app uses a browser API that you need to mock in your tests or if you just need a global setup before running your tests, add a src/setupTests.js to your project. It will be automatically executed before running your tests.

For example:

src/setupTests.js

Note: Keep in mind that if you decide to "eject" before creating src/setupTests.js, the resulting package.json file won't contain any reference to it, so you should manually create the property setupTestFrameworkScriptFile in the configuration for Jest, something like the following:

Continuous Integration

By default npm test runs the watcher with interactive CLI. However, you can force it to run tests once and finish the process by setting an environment variable called CI.

When creating a build of your application with npm run build linter warnings are not checked by default. Like npm test, you can force the build to perform a linter warning check by setting the environment variable CI. If any warnings are encountered then the build fails.

Popular CI servers already set the environment variable CI by default but you can do this yourself too:

On CI servers

Travis CI

Following the Travis Getting started guide for syncing your GitHub repository with Travis. You may need to initialize some settings manually in your profile page.

CircleCI

Follow this article to set up CircleCI with a Create React App project.

On your own environment

Windows (cmd.exe)

setCI=true&&npm test

setCI=true&&npm run build

(Note: the lack of whitespace is intentional.)

Windows (Powershell)

($env:CI=$true) -and (npm test)

($env:CI=$true) -and (npm run build)

Linux, macOS (Bash)

CI=true npm test

CI=true npm run build

The test command will force Jest to run tests once instead of launching the watcher.

If you find yourself doing this often in development, please file an issue to tell us about your use case because we want to make watcher the best experience and are open to changing how it works to accommodate more workflows.

The build command will check for linter warnings and fail if any are found.

Disabling jsdom

By default, the package.json of the generated project looks like this:

Snapshot Testing

Snapshot testing is a feature of Jest that automatically generates text snapshots of your components and saves them on the disk so if the UI output changes, you get notified without manually writing any assertions on the component output. Read more about snapshot testing.

Editor Integration

If you use Visual Studio Code, there is a Jest extension which works with Create React App out of the box. This provides a lot of IDE-like features while using a text editor: showing the status of a test run with potential fail messages inline, starting and stopping the watcher automatically, and offering one-click snapshot updates.

Debugging Tests

There are various ways to setup a debugger for your Jest tests. We cover debugging in Chrome and Visual Studio Code.

Note: debugging tests requires Node 8 or higher.

Debugging Tests in Chrome

Add the following to the scripts section in your project's package.json

This will start running your Jest tests, but pause before executing to allow a debugger to attach to the process.

Open the following in Chrome

about:inspect

After opening that link, the Chrome Developer Tools will be displayed. Select inspect on your process and a breakpoint will be set at the first line of the react script (this is done simply to give you time to open the developer tools and to prevent Jest from executing before you have time to do so). Click the button that looks like a "play" button in the upper right hand side of the screen to continue execution. When Jest executes the test that contains the debugger statement, execution will pause and you can examine the current scope and call stack.

Note: the --runInBand cli option makes sure Jest runs test in the same process rather than spawning processes for individual tests. Normally Jest parallelizes test runs across processes but it is hard to debug many processes at the same time.

Developing Components in Isolation

Usually, in an app, you have a lot of UI components, and each of them has many different states.
For an example, a simple button component could have following states:

In a regular state, with a text label.

In the disabled mode.

In a loading state.

Usually, it’s hard to see these states without running a sample app or some examples.

Create React App doesn’t include any tools for this by default, but you can easily add Storybook for React (source) or React Styleguidist (source) to your project. These are third-party tools that let you develop components and see all their states in isolation from your app.

You can also deploy your Storybook or style guide as a static app. This way, everyone in your team can view and review different states of UI components without starting a backend server or creating an account in your app.

Getting Started with Storybook

Storybook is a development environment for React UI components. It allows you to browse a component library, view the different states of each component, and interactively develop and test components.

Getting Started with Styleguidist

Styleguidist combines a style guide, where all your components are presented on a single page with their props documentation and usage examples, with an environment for developing components in isolation, similar to Storybook. In Styleguidist you write examples in Markdown, where each code snippet is rendered as a live editable playground.

Publishing Components to npm

Create React App doesn't provide any built-in functionality to publish a component to npm. If you're ready to extract a component from your project so other people can use it, we recommend moving it to a separate directory outside of your project and then using a tool like nwb to prepare it for publishing.

Making a Progressive Web App

Progressive Web Apps are faster and more reliable than traditional web pages, and provide an engaging mobile experience:

All static site assets are cached so that your page loads fast on subsequent visits, regardless of network connectivity (such as 2G or 3G). Updates are downloaded in the background.

Your app will work regardless of network state, even if offline. This means your users will be able to use your app at 10,000 feet and on the subway.

On mobile devices, your app can be added directly to the user's home screen, app icon and all. You can also re-engage users using web push notifications. This eliminates the need for the app store.

The sw-precache-webpack-plugin
is integrated into production configuration,
and it will take care of generating a service worker file that will automatically
precache all of your local assets and keep them up to date as you deploy updates.
The service worker will use a cache-first strategy
for handling all requests for local assets, including the initial HTML, ensuring
that your web app is reliably fast, even on a slow or unreliable network.

Opting Out of Caching

If you would prefer not to enable service workers prior to your initial
production deployment, then remove the call to registerServiceWorker()
from src/index.js.

If you had previously enabled service workers in your production deployment and
have decided that you would like to disable them for all your existing users,
you can swap out the call to registerServiceWorker() in
src/index.js first by modifying the service worker import:

import { unregister } from'./registerServiceWorker';

and then call unregister() instead.
After the user visits a page that has unregister(),
the service worker will be uninstalled. Note that depending on how /service-worker.js is served,
it may take up to 24 hours for the cache to be invalidated.

Offline-First Considerations

Service workers require HTTPS,
although to facilitate local testing, that policy
does not apply to localhost.
If your production web server does not support HTTPS, then the service worker
registration will fail, but the rest of your web app will remain functional.

The service worker is only enabled in the production environment,
e.g. the output of npm run build. It's recommended that you do not enable an
offline-first service worker in a development environment, as it can lead to
frustration when previously cached assets are used and do not include the latest
changes you've made locally.

If you need to test your offline-first service worker locally, build
the application (using npm run build) and run a simple http server from your
build directory. After running the build script, create-react-app will give
instructions for one way to test your production build locally and the deployment instructions have
instructions for using other methods. Be sure to always use an
incognito window to avoid complications with your browser cache.

If possible, configure your production environment to serve the generated
service-worker.jswith HTTP caching disabled.
If that's not possible—GitHub Pages, for instance, does not
allow you to change the default 10 minute HTTP cache lifetime—then be aware
that if you visit your production site, and then revisit again before
service-worker.js has expired from your HTTP cache, you'll continue to get
the previously cached assets from the service worker. If you have an immediate
need to view your updated production deployment, performing a shift-refresh
will temporarily disable the service worker and retrieve all assets from the
network.

Users aren't always familiar with offline-first web apps. It can be useful to
let the user know
when the service worker has finished populating your caches (showing a "This web
app works offline!" message) and also let them know when the service worker has
fetched the latest updates that will be available the next time they load the
page (showing a "New content is available; please refresh." message). Showing
this messages is currently left as an exercise to the developer, but as a
starting point, you can make use of the logic included in src/registerServiceWorker.js, which
demonstrates which service worker lifecycle events to listen for to detect each
scenario, and which as a default, just logs appropriate messages to the
JavaScript console.

By default, the generated service worker file will not intercept or cache any
cross-origin traffic, like HTTP API requests,
images, or embeds loaded from a different domain. If you would like to use a
runtime caching strategy for those requests, you can eject
and then configure the
runtimeCaching
option in the SWPrecacheWebpackPlugin section of
webpack.config.prod.js.

Progressive Web App Metadata

The default configuration includes a web app manifest located at
public/manifest.json, that you can customize with
details specific to your web application.

When a user adds a web app to their homescreen using Chrome or Firefox on
Android, the metadata in manifest.json determines what
icons, names, and branding colors to use when the web app is displayed.
The Web App Manifest guide
provides more context about what each field means, and how your customizations
will affect your users' experience.

Analyzing the Bundle Size

Source map explorer analyzes
JavaScript bundles using the source maps. This helps you understand where code
bloat is coming from.

Then to analyze the bundle run the production build then run the analyze
script.

npm run build
npm run analyze

Deployment

npm run build creates a build directory with a production build of your app. Set up your favorite HTTP server so that a visitor to your site is served index.html, and requests to static paths like /static/js/main.<hash>.js are served with the contents of the /static/js/main.<hash>.js file.

Static Server

For environments using Node, the easiest way to handle this would be to install serve and let it handle the rest:

npm install -g serve
serve -s build

The last command shown above will serve your static site on the port 5000. Like many of serve’s internal settings, the port can be adjusted using the -p or --port flags.

Run this command to get a full list of the options available:

serve -h

Other Solutions

You don’t necessarily need a static server in order to run a Create React App project in production. It works just as fine integrated into an existing dynamic one.

The choice of your server software isn’t important either. Since Create React App is completely platform-agnostic, there’s no need to explicitly use Node.

The build folder with static assets is the only output produced by Create React App.

However this is not quite enough if you use client-side routing. Read the next section if you want to support URLs like /todos/42 in your single-page app.

Serving Apps with Client-Side Routing

If you use routers that use the HTML5 pushState history API under the hood (for example, React Router with browserHistory), many static file servers will fail. For example, if you used React Router with a route for /todos/42, the development server will respond to localhost:3000/todos/42 properly, but an Express serving a production build as above will not.

This is because when there is a fresh page load for a /todos/42, the server looks for the file build/todos/42 and does not find it. The server needs to be configured to respond to a request to /todos/42 by serving index.html. For example, we can amend our Express example above to serve index.html for any unknown paths:

Now requests to /todos/42 will be handled correctly both in development and in production.

On a production build, and in a browser that supports service workers,
the service worker will automatically handle all navigation requests, like for
/todos/42, by serving the cached copy of your index.html. This
service worker navigation routing can be configured or disabled by
ejecting and then modifying the
navigateFallback
and navigateFallbackWhitelist
options of the SWPreachePluginconfiguration.

When users install your app to the homescreen of their device the default configuration will make a shortcut to /index.html. This may not work for client-side routers which expect the app to be served from /. Edit the web app manifest at public/manifest.json and change start_url to match the required URL scheme, for example:

"start_url":".",

Building for Relative Paths

By default, Create React App produces a build assuming your app is hosted at the server root.
To override this, specify the homepage in your package.json, for example:

"homepage":"http://mywebsite.com/relativepath",

This will let Create React App correctly infer the root path to use in the generated HTML file.

Note: If you are using react-router@^4, you can root <Link>s using the basename prop on any <Router>.
More information here.

Serving the Same Build from Different Paths

Note: this feature is available with react-scripts@0.9.0 and higher.

If you are not using the HTML5 pushState history API or not using client-side routing at all, it is unnecessary to specify the URL from which your app will be served. Instead, you can put this in your package.json:

"homepage":".",

This will make sure that all the asset paths are relative to index.html. You will then be able to move your app from http://mywebsite.com to http://mywebsite.com/relativepath or even http://mywebsite.com/relative/path without having to rebuild it.

Install the Firebase CLI if you haven’t already by running npm install -g firebase-tools. Sign up for a Firebase account and create a new project. Run firebase login and login with your previous created Firebase account.

Then run the firebase init command from your project’s root. You need to choose the Hosting: Configure and deploy Firebase Hosting sites and choose the Firebase project you created in the previous step. You will need to agree with database.rules.json being created, choose build as the public directory, and also agree to Configure as a single-page app by replying with y.

=== Project Setup
First, let's associate this project directory with a Firebase project. You can create multiple project aliases by running firebase use --add, but for now we'll just set up a default project.
? What Firebase project do you want to associate as default? Example app (example-app-fd690)
=== Database Setup
Firebase Realtime Database Rules allow you to define how your data should be
structured and when your data can be read from and written to.
? What file should be used for Database Rules? database.rules.json
✔ Database Rules for example-app-fd690 have been downloaded to database.rules.json.
Future modifications to database.rules.json will update Database Rules when you run
firebase deploy.
=== Hosting Setup
Your public directory is the folder (relative to your project directory) that
will contain Hosting assets to uploaded with firebase deploy. If you
have a build process for your assets, use your build's output directory. ? What do you want to use as your public directory? build ? Configure as a single-page app (rewrite all urls to /index.html)? Yes ✔ Wrote build/index.html i Writing configuration info to firebase.json... i Writing project information to .firebaserc... ✔ Firebase initialization complete!

IMPORTANT: you need to set proper HTTP caching headers for service-worker.js file in firebase.json file or you will not be able to see changes after first deployment (issue #2440). It should be added inside "hosting" key like next:

Step 3: Deploy the site by running npm run deploy

Step 4: Ensure your project’s settings use gh-pages

Finally, make sure GitHub Pages option in your GitHub project settings is set to use the gh-pages branch:

Step 5: Optionally, configure the domain

You can configure a custom domain with GitHub Pages by adding a CNAME file to the public/ folder.

Notes on client-side routing

GitHub Pages doesn’t support routers that use the HTML5 pushState history API under the hood (for example, React Router using browserHistory). This is because when there is a fresh page load for a url like http://user.github.io/todomvc/todos/42, where /todos/42 is a frontend route, the GitHub Pages server returns 404 because it knows nothing of /todos/42. If you want to add a router to a project hosted on GitHub Pages, here are a couple of solutions:

You could switch from using HTML5 history API to routing with hashes. If you use React Router, you can switch to hashHistory for this effect, but the URL will be longer and more verbose (for example, http://user.github.io/todomvc/#/todos/42?_k=yknaj). Read more about different history implementations in React Router.

Alternatively, you can use a trick to teach GitHub Pages to handle 404 by redirecting to your index.html page with a special redirect parameter. You would need to add a 404.html file with the redirection code to the build folder before deploying your project, and you’ll need to add code handling the redirect parameter to index.html. You can find a detailed explanation of this technique in this guide.

Troubleshooting

"/dev/tty: No such a device or address"

If, when deploying, you get /dev/tty: No such a device or address or a similar error, try the follwing:

"Module not found: Error: Cannot resolve 'file' or 'directory'"

It means you need to ensure that the lettercase of the file or directory you import matches the one you see on your filesystem or on GitHub.

This is important because Linux (the operating system used by Heroku) is case sensitive. So MyDirectory and mydirectory are two distinct directories and thus, even though the project builds locally, the difference in case breaks the import statements on Heroku remotes.

"Could not find a required file."

If you exclude or ignore necessary files from the package you will see a error similar this one:

Advanced Configuration

You can adjust various development and production settings by setting environment variables in your shell or with .env.

Variable

Development

Production

Usage

BROWSER

✅

❌

By default, Create React App will open the default system browser, favoring Chrome on macOS. Specify a browser to override this behavior, or set it to none to disable it completely. If you need to customize the way the browser is launched, you can specify a node script instead. Any arguments passed to npm start will also be passed to this script, and the url where your app is served will be the last argument. Your script's file name must have the .js extension.

HOST

✅

❌

By default, the development web server binds to localhost. You may use this variable to specify a different host.

PORT

✅

❌

By default, the development web server will attempt to listen on port 3000 or prompt you to attempt the next available port. You may use this variable to specify a different port.

HTTPS

✅

❌

When set to true, Create React App will run the development server in https mode.

PUBLIC_URL

❌

✅

Create React App assumes your application is hosted at the serving web server's root or a subpath as specified in package.json (homepage). Normally, Create React App ignores the hostname. You may use this variable to force assets to be referenced verbatim to the url you provide (hostname included). This may be particularly useful when using a CDN to host your application.

CI

🔶

✅

When set to true, Create React App treats warnings as failures in the build. It also makes the test runner non-watching. Most CIs set this flag by default.

REACT_EDITOR

✅

❌

When an app crashes in development, you will see an error overlay with clickable stack trace. When you click on it, Create React App will try to determine the editor you are using based on currently running processes, and open the relevant source file. You can send a pull request to detect your editor of choice. Setting this environment variable overrides the automatic detection. If you do it, make sure your systems PATH environment variable points to your editor’s bin folder. You can also set it to none to disable it completely.

CHOKIDAR_USEPOLLING

✅

❌

When set to true, the watcher runs in polling mode, as necessary inside a VM. Use this option if npm start isn't detecting changes.

GENERATE_SOURCEMAP

❌

✅

When set to false, source maps are not generated for a production build. This solves OOM issues on some smaller machines.

NODE_PATH

✅

✅

Same as NODE_PATH in Node.js, but only relative folders are allowed. Can be handy for emulating a monorepo setup by setting NODE_PATH=src.

Troubleshooting

npm start doesn’t detect changes

When you save a file while npm start is running, the browser should refresh with the updated code.
If this doesn’t happen, try one of the following workarounds:

If your project is in a Dropbox folder, try moving it out.

If the watcher doesn’t see a file called index.js and you’re referencing it by the folder name, you need to restart the watcher due to a Webpack bug.

Some editors like Vim and IntelliJ have a “safe write” feature that currently breaks the watcher. You will need to disable it. Follow the instructions in “Adjusting Your Text Editor”.

If your project path contains parentheses, try moving the project to a path without them. This is caused by a Webpack watcher bug.

If the project runs inside a virtual machine such as (a Vagrant provisioned) VirtualBox, create an .env file in your project directory if it doesn’t exist, and add CHOKIDAR_USEPOLLING=true to it. This ensures that the next time you run npm start, the watcher uses the polling mode, as necessary inside a VM.

If none of these solutions help please leave a comment in this thread.

There are also reports that uninstalling Watchman fixes the issue. So if nothing else helps, remove it from your system and try again.

npm run build exits too early

It is reported that npm run build can fail on machines with limited memory and no swap space, which is common in cloud environments. Even with small projects this command can increase RAM usage in your system by hundreds of megabytes, so if you have less than 1 GB of available memory your build is likely to fail with the following message:

The build failed because the process exited too early. This probably means the system ran out of memory or someone called kill -9 on the process.

If you are completely sure that you didn't terminate the process, consider adding some swap space to the machine you’re building on, or build the project locally.

npm run build fails on Heroku

This may be a problem with case sensitive filenames.
Please refer to this section.

This will only work for locales that have been explicitly imported before.

npm run build fails to minify

Some third-party packages don't compile their code to ES5 before publishing to npm. This often causes problems in the ecosystem because neither browsers (except for most modern versions) nor some tools currently support all ES6 features. We recommend to publish code on npm as ES5 at least for a few more years.

To resolve this:

Open an issue on the dependency's issue tracker and ask that the package be published pre-compiled.

Note: Create React App can consume both CommonJS and ES modules. For Node.js compatibility, it is recommended that the main entry point is CommonJS. However, they can optionally provide an ES module entry point with the module field in package.json. Note that even if a library provides an ES Modules version, it should still precompile other ES6 features to ES5 if it intends to support older browsers.

Fork the package and publish a corrected version yourself.

If the dependency is small enough, copy it to your src/ folder and treat it as application code.

In the future, we might start automatically compiling incompatible third-party modules, but it is not currently supported. This approach would also slow down the production builds.

Alternatives to Ejecting

Ejecting lets you customize anything, but from that point on you have to maintain the configuration and scripts yourself. This can be daunting if you have many similar projects. In such cases instead of ejecting we recommend to forkreact-scripts and any other packages you need. This article dives into how to do it in depth. You can find more discussion in this issue.